Craniofacial defects result from disruptions of the normal mechanisms that control head development. Thus understanding the basis for inherited craniofacial disorders requires knowledge of the embryonic molecular and genetic processes that direct pattern formation in the head. These include the tissue interactions that establish skeletogenic mesenchyme, its condensation and differentiation into cartilages and bones, and their interconnections to form a functional skeletal network. This proposal examines these issues using the genetics and embryological advantages of zebrafish as a model. Zebrafish embryos form a simple, segmentally organized pharyngeal skeleton, where homologies with human skeletal elements are recognizable, and many of the genes that pattern these segments have been conserved between fish and humans. It is thought that a major component of patterning within each pharyngeal segment is endothelin-1 (Et-1), a signal that directs patterns of chondrogenesis and osteogenesis. Experiments are proposed to dissect the role of this signal using a set of zebrafish mutants including one mutant in Et-1 itself as well as mutants and antagonists of Et receptors. Aim 1 is to characterize genes of the ephrin and eph receptor tyrosine kinase families that we have implicated in controlling neural crest morphogenesis and their responses to Et-1. Aim 2 is to analyze one mutant called schmerle, which phenotypically resembles loss of Et-1 function, in more detail and to clone its underlying genetic basis. Aim 3 focuses on defining which specific tissue interactions require Et-1, between either the pharyngeal epithelium or mesoderm and the skeletogenic neural crest, by using cell transplantation in Et-1 mutants to dissect the biochemical basis for skeletal patterning.